Magnetic core having discrete bends at each corner



Feb. 28, 1967 B. B. ELLIS 337132 MAGNETIC CORE HAVING DISCREIE BENDS AT BACH CORNER Original Filed 0c1. 10, 1963 Sheecs-Sheet 2 production.

United States Patent O 3307,132 MAGNETIC CORE HAVING DISCRETE BENDS AT EACH CORNER Belvin B. Ellis, Pulaski, Pa., assignor t Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation 0f Pennsylvania Continuation of abandoned application Sex. N0. 315,300, Oct. 10, 1963. This application May 13, 1966, Set. N0. 559,668

14 Claims. (C1. 336211) This application is a continuation of application Serial N0. 315,300 filed October 10, 1963, now aba-ndoned, which was assigned to the same assignee as this application.

This invention relates in general to electrical inductive apparatus, and more particularly to magnetic core structures for inductive apparatus.

Wound magnetic core structures of the prior art, such as those shown and described in U.S. Patents 2972804 and 2973494 granted February 28, 1961, and assigned to the same assignee as the present application, have two yoke por-t-i0ns and 2 leg portions circurnscribing a generally rectangula-r window or opening. The cores generally have ronnded corners on the external periphery. The yoke and leg portions Which circumscribe the window o1 opening in the magnetiocore should have straight sides and the core window should have well formed sharp corners to allow associated coils to properly fit the core legs and window.

In an attempt maintain the desired or as-formed shape of the magnetic core during and after the stress annealing operation to rernove the stresses induced by the forming operation, it is necessary to resort to expensive alloy steel holdin:g fixtures, such as caps and plates. Upper and lower caps locate and hold the outer periphery of the core, as well as the core window. The plates extend from the upper to the lower cap, along the legs cf the magnetic core. Bach magnetic core size requires its 0Wn complete st of caps and plates, which is an added expense each time a new eure design is put into Further, maintenance of the caps to maintain them in the desired shape after repeated heating and cooling eycles is extensive.

The added expense of the caps and plates and the resulting design infiexibility once the fixtures are 0btained, are not the only problems associated with the prior art design. Even using the fixtures 01' supports,

-the round outer corners of the magnetic core cause a oertain amoun-t of spring-back, resulting in bowing of the core legs, rounding of the c0re ends and rounding of the inner corners of the core windows. The spring-back problern is a=ggrawated by the continual usage of the same holding fixtunes, as they are warped by the thermal cycling they are subjected to.

Any change in the core shape after the anneal cycle from the position it occupies during the anneal, or stressing the cores in any way, substantially increases the core losses. Bowing of the core legs causes stress to be produced in the core by pressure fro-m end frarnes, thus increasing the core losses. Bowing or rounding of the core ends allows laminations to Slip aronnd the corners during assembly with the associated eleetrical coil, into positions otheir than those occupied during the stress relief, thus stressing the larninat-ions and increasing the core losses. Rounded inner corners prevent t-ighter design or close fit of cores and coils, thus the coils do not properly fit the core legs and windows. Also, on core designs using a lapped joi-nt closure, such as the stepped design taught by the U.S. patents hereinbefore referred to, incomplete closure causes joint losses to be substantially increased.

Another disadvantage of the holding fixtures is the 3307132 Patented Feb. 28, 1967 fast that the annealing furnace must be sized larger than dictated by the thermal requirements of the cores in order to heat the mass of the holding fixtures. Further, the caps shield the cores from heat radiation during the annealing process, requiring a longer heat treating cycle to get the cores heated uniforrnly to the desired annealing iemperature.

Accordingly, -it is an object of this invention to provide a new and improved magnetic core structure for electrical inductive apparatus.

Another object of the invention is to provide a new and improved magnetic core structure that Will mainta-in the desired shape before and after the annealing cycle.

A further object of the invention -is to provide a new and improved magnetic core structure that can be assembled around its associated coil without the laminations assuming positions diflerent from those occupied during the annealing cycle.

Still another object of the invention is to provide a new and improved magnetic core str-ucture that can be assembled with its associated coil without substantially increasing the losses of the magnetic core.

Still a further object of the invention is to provide a new and improved magnetic core structure that will allow its associated coil t0 closely and accurately fit the core legs and window.

Still another object of the invention is to provide a new and improve-d magnetic c ore structure that Will maintain its desired shape after annealing without the necessity of ntilizing external supporting fixtures, and produce a core hav.ing a minimum of losses.

Briefly, the present invention accomplishes the abovecited objects by providing a polygonal magnetic core having 4 major sides and a plurality of die-formed bends, preferably 2 or 3 at the corners instead of round corners, which provides a plurality of additional or minor sides. One bend per corner would have the disadvantage of increasing the weight of average size cores by 6 to 7% thus increasin=g the cost of the cores as well as increasing the core losses. By using two or three bends per corner, substantially n0 increase in core material is required, as the core corners closely conform to round corners. The angles of all of the bends of the corners are alike, and sharp enough to eliminate bowing and spring-back as a problem. The inner corners of the core w indow are square and sharp and accurately diepositioned. After die-forming, the core legs are slightly bowed-in instead of be ing bowed-out as in the prior art, thus the only fixture required during the heat treating cycle is a spreader type fixt-ure in the internal core window. It is unnecessary to use expensive external and in-ternal supports such as caps and plates. The sharp bends at the corners canse all of the laminations to line up properly after assembling the coil on the cone, thus eliminating slipping or incomplete closure of the joint in the core. Further, since the eure is die-formed, the coils may be assernbled quickly and accura-tely without stressing the core and increasing its losses. The absence of external fixtures allows the cores to be more rapidly heated to the annealing -temperature, and reduces the furnace size necesasry to heat the cores, as there are no massive external fixtures to also heat -to the annealing temperature.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty Which characterize the invention will be pointed out in particularity in the cla ims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had t0 the accompauying drawings in which:

FIGURE 1 is a view in end elevation f. a wound core having straight core legs and a window Witl1 square and sharp corners;

FIG. 2 is a; view in end elevation of a wound co-re, before and after anneal, which had not had the benefit of external supporting fixtures;

FIG. 3 is a pictorial view of a wound core with external supporting fixtures;

, FIG. 4 is a view in end elevation of a wound core after anneal which had been externally supported in a fixture similar to the one shown in FIG. 3;

FIG. 5 is a view in end elevation of a die-formed wound type core after anneal, having straight legs and a window having square, sharp corners;

FIG. 6 is a view in end elevation of a wound type core after die-forming and before anneal, having a plurality of bends at the core corners and the core legs bowed slightly inward;

FIG. 7 is a view in side elevation, partially in phantom showing the wound type core of FIGURE 5 prior to anneal, with a spread'er type intemal fixture;

FIG. 8 is a fragmentary view of the end elevation of a wound type core, having three bends at each corner;

FIG. 9 is a fragmentary view of the end elevation of a wound type core having one bend at each corner;

FIG. 10 is a fragmentary view of the end elevation of a wound type core showing how the joints of some groups of larninations may be disposed t0 coincide with one of the corner bends to lock the core together;

FIG. 11 is a fragmentary view, in cross section, of the end elevation of wound type cores and associated coils, showing how the die-formed cores could allow a predetermined ainount of space to be provided for voltage cleara1'1ce from the core end to the inner high voltage Winding on the coil;

FIG. 12 is a fragmentary view of the end elevation of a wound type core showing that individual groups 01 laminations may be cut or sheared separately and then bent and put together in a composite core structure;

FIG. 13 is a fragmentary view of the end elevation of a wound type core, illustrating how individual laminations can be projected up and around a core end by shifting one lamination from each group of laminations;

FIG. 14 is a pictorial view of a magnetic core in one of the steps prior to die-forming;

FIG. 15 is a view in end elevation of a wound type core showing a clamp adjacent to a cut;

FIG. 16 is a view in end elevation of the core shown in FIG. 15, With a rnain portion of the core straightened to obtain the laminations in stepped relationship;

FIG. 17 is a view in end relation 0f the wound type core shown in FIG. 16 curled up t0 form a substantially cir cular core With the laminations forming stepped-lapped joints;

' FIG. 18 is a view in end elevation of a wound and split core showing the laminations separated into groups With the ends disposed in stepped relationship;

FIG. 19 is a view in end elevation of a core showing the laminations matching as they are brought end to end;

FIG. 20 is a view in end elevation of a wound type core illustrating joints disposed in more than one location throughout the core;

FIG. 21 is a pictorial view illustrating a magnetic core being formed in a die;

FIG. 22 is a front elevation showing how the individual laminations of a wound type core may be sheared to the proper length preparatory to forming the magnetic core structure.

Referring now to the drawings, and FIG. 1 in particular, there is shown a front elevation of a typical wound core 20 of the prior art, having legs 22 and 24 separated by upper and lower yokes 01' ends 26 and 28, respectively. The core 20 may have a joint 30, in one of the legs of the core 20, such as in leg 24 as shown, or in the upper or lower yoke portions 26 and 28, t0 facilitate introduction of the associated electrical coil. The joint 30 may be of the stepped lap design as shown, which is described in the U.S. patents hereinbefore referred to, a butt-lap joint, or any other type of joint.- The core shown in FIGURE 1 is of the desired shape toinsure that the core Will have the lowest possible losses, having straight legs 22 and 24, and a window 32 formed by legs 22 and 24 and yokes 26 and 28 which has square sharp corners 34. However, it is extremely ditficult to form a core having the configuration 0f FIGURE l, and have it maintain its as-formed sonfiguration without the aid of external holding means. The wund corners 36 01 core 20 cause the core to spring back after forming, and assume a shape similar to that shown in FIGURE 2. T he legs 22 and 24 of core 20 are bowed outward, and the window 32 is poorly shaped, with the corners 34 of the window 32 becorning rounded. In order to prevent the cores from taking the shape shown in FIGURE 2, it is necessary to use some type of external holding fixture such as shown in FIG. 3.

FIGURE 3 illustrates one of the prior art methods of holding cores during the stress annealing operation. Upper and lower alloy steel caps 40 and 42, respectively, and side plates 44 position the outer periphery and window of magnetic core 46 and hold the core during the heat treating cycle. As can be readily seen from FIG. 3, the holding fixture shields the core from radiant heat during the stress annealing cycle, requiring a much longer time t0 bring the core up to the proper temperature. Also, the mass of the fixtures has to be heated, along With the cores, thus requiring a larger furnace capacity than would be required for heating the cores alone.

However, in spite o f elaborate and expansive holding fixtures, cores still tend to take a shape after anneal similar to the eure shown in FIGURE 4. The spring-back in the core 50 still produces a bow in the legs 52 and 54 and the ends 53 and 55, and the corners 56 of the core window are not square and sharp. This condition is further aggravated by the warpage and growth of the heat treating fixtures after they have been subjected t0 repeated stress annealing heating and cooling cycles. Further, all cores do not deviate from the desired shape in the same degree, so there is no uniforrnity in the configuration 0f the cores In order t0 manufacture the coils so they will fit all of the cores, they must necessarily be made so they Will fit the worst condition of core deviation. This causes poor fits between the majority of cores and coils between the core legs and Windows, resulting in increased losses. Bowed core legs 'cause stresses to be produced in the core from pressure from end frames and coils, which further adds to the core losses. The rounding or bowing of the core ends allows larninations to slip during assembly from positions other than those occupied during the stress anneal, causing the larninations to be stressed, and increase the c0re losses. The core joints are difficult to close aiter assembly with the coil so that the desired amount of overlap is seldom maintained, adding to the losses of the core joint. The rounded window corners prevent close fit of the coils and cores, adding still further t0 the core losses.

A core design which eliminates the requirernent of expensive external holding fixtnres during the heat treating cycle, and which produces a core having straight legs and yokes, a window having square sharp corners, and a core that has a minimum of losses, With0ut substantially increasing the core weight, is shown in FIG. 5. Tl1e polygonal core design shown in FIG. 5 is die-formed by a method which will be described in detail hereinafter, so there Will be substantially n0 deviation in core configuration from one core to the next. More specifically, the polygonal core 60 has 4 major sides, 61, 63, 65 and 67; and is die-formed to create a plurality of discrete bends 62 and 64 at each corner. Thus, a plurality of smaller or minor sides 69, '71, 73 and 75 are formed. The springback in the cores of the prior art is caused by the rounded corners used in corestructures of the prior art. Pro gressively, from the inside to the outside, there is an in.

creasing radius on the corners that causes increasing spring-back on the laminations. The larger the radius, the greater the spring-back tendency. A round corner has an infinite number of bends, With no discrete bends. The minirnum spring-back would be the situation where each corner has one discrete bend, as shown in the fragmentary view in FIG. 9. However, one bend per corner would cause a 6 to 7% increase in the weight cf the core, causing =a corresponding increase in the cost of the material for each core, as well as increasing the core losses. The material shown in the cross-hatched area 65 of FIGURE 9 would all be excess material.

FIG. 5 illustrates a polygonal core 60 having two discrete bends, 62 and 64, per corner, which causes the core corners to closely conform in sh=ape to round corners, thus wasting a negligible arnonnt of material and still providing a core which has substantially no tendency to spring-back. It has been found desirable to actually give the legs of the die-formed core a reverse bend during the die-forming operation, as shown in FIGURE 6. Fig. 6 shows a core 70 in which the legs 72 and 74 bow slightly inward. Then, just prior to the stress annealing cycle, a simple, internal spreader type fixture may be added to the core window to str-aighten the core legs, such as the spreader fixture shown in FIG. 7. FIG. 7 shows the end view of a core 80, With a spreader fixture 82 being shown in position in the core window With broken lines. The core stresses are thus relieved during stress anneal while pressing against the spreader fixture 82, producing a core having the appearance of core 60 shown in FIGURE 5. EX- pensive external caps and plates, as shown in FIG. 3, are unnecessary.

A core having two discrete bends per corner may have one bend formed 225 degrees from the vertical and the other 22.5 degrees from the horizontal, as shown in FIG- URE 5. If three discrete bends per corner =are utilized, one bend may be made at 45 degrees, another at 15 degrees from the vertical and the remaining band 15 degrees from the horizontal, such as shown in FIG. 8. Although more discrete bends than 3 per corner may be utilized, it is not considered practical since the minor savings in material are more than olfset by the increased -amount of spring-back introduced by additional bends.

Although, as hereinbefore stated, any type of core joint may be utilized, the improved core structure disclosed herein is particularly well suited to the stepped-lap type jo-int, which Will be described in detail hereinafter. When the stepped-lap type joint is used, s0me step-lap groups of laminations may be brought around the corner to cross one or more of the corner bends, as shown in FIG. 10, thus locking the core together. More specifically, FIG. 10 is a fragmentary view showing the corner of a core 90 which has bends 92 and 94. The thin, pencil-like extensions 96 represent groups of laminations, With each group of laminations being step-lapped. The end groups of larninations are shown crossing the bends 92 and 94, which provide a locking influence upon the joint, preventing it from opening after it has been disassernbled and reassembled through the window of its associated electrical coil.

FIG. 11 shows a portion of two magnetic cores and high and low voltage windings, in seotion, illustrating an embodiment of the invention whereby voltage clearance is provided from the core end to the high voltage winding on the core. More specifically, magnetic cores 100 and 102 are shown in assembled relationship with high voltage winding 104, and inner and outer low voltage Windings 106A and 106B, respectively. A clearance o1 gap 110 is maintained between core 100 and high voltage Winding 104 by shaping the portion of the die that forms the window such that the bends 112 and 114 do not come to a point at the window corner, but leave a space 116 which causes a polygonal window to be forrned. Since the cores are die-formed, this space 110 is accurate and repeatable from core to core. Thus the coil may be manufactured to closely fit the core, and the coil height may be such that the gap formed by leaving space 116 between the bends 112 and 114 Will provide the necessary voltage clearance.

Instead of die-forming the complete core in one operation, individual groups of laminations may be formed separately and then assembled so the bends in the corners line up. FIGURE 12 is a fragmentary view illustrating how a core corner would look when lamination groups are separately bent and then assembled. The substantially triangul-ar shaped openings 122 are advantageous, as they provide some looseness or tolerance during manufacture and assembly. A method of providing tolerances or looseness during assembly of a core which is die-formed all at once, is to place spacers made of paper or other combustible material between the groups of larninations such that they Will apmar in the proper locations at the end of the core after shaping. The spacers Will burn out during the stress anneal cycle to provide spaces or looseness during assernbly.

FIGURE 13 is a fragmentary view of the corner of a core illustrating another embodiment of the invention which Will aid in locking the core after assembly and eliminate the necessity of a core band. The end of one lamination 132 from each group of step-lapped laminations may be shifted around a core end so that the shifted larnination crosses the point where one of the sharp bends is to be made. The sharp bend in the lamination from each group of laminations locks the core joints securely in position.

A method of producing magnetic cores embodying the principles of this invent-ion, such as the core shown in FIG. 5 is to Wind a plurality 0f turns 151 of oriented magnetic strip material upon one another, as shown in FIGURE 14 to form a core 149. The number of turns 151 and the Width of the strip will depend upon the design and capacity of the core. The diameter of the core 149 Will be largely determined by the size of the preforrned coil or Winding With which it is to be utilized. After the core 149 is wound, it is back wound to provide the necessary looseness, and to provide the desired internal and external diameters. The core 149 is then out, either perpendicular to the side of the strip along solid line 153 as shown, or at an oblique angle thereto, such as along dotted line 146. The oblique angle serves to reduce the magnetic induction at the joint by increasing the joint length.

If a stepped-lap 10W loss joint is desired, wound core 149, as shown in FIGURE 15, may have its laminations 151 cl'amped in fixed relationship to one another on one side of the out 153 by rneans -of a clarnp 155. After the clamp 155 has been applied, the laminations of the core are straightened out, as shown generally at 157 in FIG- URE 16. It will be obvious that the straightening out of the main portion of the core will 'arrange the laminations 151 in stepped relationship as shown at 159.

If it is desired to step the laminations more in one operation than effected by straightening out the group or stack of laminations, they may be bowed or curved in the opposite direction from the curve in the first Winding. Other operations of this type may be introduced to acceler'ate the making of the core. Such operations will be obvious from the description of the present method.

A second clamp 161 is now applied to the other end of the straight seotion 157 of the core. T he clamp 155 is rele'ased and the straight section of the core 157 curled up to form a loop as shown in FIGURE 17. As the laminations curl up, the portions of the laminations previously in the straight section 157 will move relative to one another and become disposed in stepped relationship to match the stepped laminations shown generally at 159. This rolling out operation f-or arranging lamination ends in stepped arrangement may be repeated a number of times to get the stepped relationship required. For example, when the entire core build-up of laminations is rolled out at once, it is po ssible to get the end portion stepped to the extent of substantially six times the thicknes of the laminations by repeating the operation twice. If the laminations are separated into a plurality of groups, a six to one ratio f steplength to l'arnination thickness may be obtained by rolling out each group once. Also, instead of utilizing a complete roll out, it is possible to obtain the steplength to lamination thickness ratio desired by using a plurality of partial rollouts. By positioning clamp 161 closer to the -cut 153, an even greater amount of steplength per rollout may be achieved.

If now the stepped ends of the laminations are fitted together, a series of stepped-lap joints Will be formed in the laminations, as shown generally at 163. As Will be observed, the laminations are disposed With each lamination forming a joint which is ofiset or staggered relative to the joint in the next adjacent lamination. The end of one lamination laps the end 0f the next adjacent lamination. Accordingly, the joint comprises a number of laminations, the ends of which are arranged in a stepped order, the end portions of one lamination lapping the end portion of the next lamination forming a joint which may be descri-bed as a stepped-lap joint bec'ause of the arrangement of the laminations.

For best results, the ends of the individual laminations should abut one another. The percentage of joint loss caused by space between the lamination ends, however, is a function of the steplength to l'amination thickness. For exarnple, With a 2 /2 to 1 ratio, which is the minimum ratio recornrnended, it is very important that the laminations abut -one another, as any space between lamin-ation ehds causes a substantial increase in joint losses. With a ratio of 6 to 1, the space between lamination ends may be as long as the thickness 0f one or two laminations Wit-hout substantially increasing the core losses. It is sometirnes desirable to go to ratio of approximately 20 to 1, which 'allows a space the length of several lamination thicknesses without substantially increasing joint losses.

The annealing operation may -be performed after the core has been shaped, as shown in FIGURE 6, and no further operations are to be perforrned which will introduce stresses in the iron 0f the core and, therefore, in crease the core losses. After the core has been shaped, With four major sides and a plurality of minor sides created by two or more discrete bends as described herein, and annealed With a simple spreader fixture disposed in the core window, the joint may be opened and the laminations fitted through the preformed coil and fitted together to form the stepped-lap joint described. It has been found that the bending of the laminations to fit them around the coil is not severe enough to appreciably strain the material. Therefore, the losses of the core in operation are not greatly increased by the assembly Operation.

FIGURE 18 illustrates how laminations may be arranged in groups. In this particular illustration, there are four groups, 171, 173, 175 and 177 shown. Group 171 has only two laminations, group 173, three; group 175, four; and group 177, five. However, this is only for comvenience and illustration. The groups 171 and 177, inclusive, many inclu-de as many individual laminations as the designer may find preferable for bis building operations and final product. In practice, it is cornrnon to utilize groups having from 15 to 30 laminations.

Since the laminations are all of the same gauge, standard jigs and devices may be made which will readily separ'ate the desired nurnber of laminations required for each group frorn the body of the core. When the laminations have been separated into the groups of a desired size, they may be manipulated in the manner described hereinbefore, either as individual groups, or as a whole to arrange the individual laminations in stepped relationship.

If the groups of laminations are arranged in individual groups to get the required stepped relationship of the individual laminations, they may be assembled by superimposing the groups on one another in the relationship in which they were wound. If all the groups have been arranged while held together but With the groups kept separated by a jig or other device, then they are ready to be adjusted to locate the joints as desired.

Assuining now that the groups have been arranged to get the laminations 151 in the required stepped relationship as individual group-s, then group 173 Will be superimposed upon group 171 when the stepped-lapped joints of the groups are substanitally aligned. T he same practice will be followed With groups 175 and 177. The result is that the stepped-lapped joints Will all now be 10- c'ated so that when the core is shaped as shown in FIG- URE 5, the stepped-lapped joints lie Within a straight section of the core. This greatly facilitates holding the rnembers which together malte the joint, and results in a 10W loss joint.

Instead of having the joints of groups 171, 173, 175 and 177 line 11p as shown in FIGURE 19, it may be desirable to arr'ange the groups so that the joints Will fall Within two or more 0f the straight portions of the core after die-forming. F01 example, FIGURE 20 illustrates a polygonal core 191 in which stepped-lapped joints 193 and 195 are disposed on legs 197 and 199, respectively. Distributing the joints into a plurality of locations m'akes the core 191 more compact, as the core doesnt completely open at any one location. An alternative method of f0rrning the core With the joints already distri-buted to different predetermined positions in the core, is to form the core With the joints as shown in FIGURE 15, and then alter dis'assembling the core in groups of laminations, and the desired joint pattern is formed in each group, reassembling the core With the joints of a first portion of groups in one location and the joints of a second portion of groups in another location. If the angle of the out through the core was oblique to the edge 01 the strip, as hereinbefore described, the groups of lamin'ations could be turned over 0n alternate groups so that the oblique cuts on adjacent groups are oppositely slanted. The assembled core would appear sirnilar to the core shown in FIGURE 19, except joints would also appear in sorne other predetermined portion.

The core 149, as shown in FIGURE 19, is then dieformed or shaped to obtain the discrete sharp bends at each corner, in a die arrangement such as shown in FIG- URE 21. More specifically, FIGURE 21 is a pictorial view of a die arrangernent having upper and lower sections, and 152, respectively, with inner mandrel or internal die 154. In practice, a three piece inner die 154 has been found to produce excellent results. The inner piece of the die is slightly wedge shaped to produce an endwise rnovement of die 154 during the forming operation. Other internal die arrangements, however, may be equally suitable. The core 142 is placed between the upper and lower die sections 150 and 152 when the die sections 150 and 152 are in an open position. The die sections are then closed by any appropriate means, such as a hydraulic press. At an appropriate tirne during the forming of core 142, the inner die mandrel 154 is inserted into the opening of core 142, which forms accurate, sharp corners in the core window. The upper and lower die sections 150 and 152 are appropriately shaped to form the desired number of sharp bends and 162 at each corner of the core 142. The laminations seem to flow or extrude into the corners towards the end of the forrning cycle forming well defined sharp bends 160 and 162. The upper and lower die sections are designed With a slight curve at locations 164 and 166, and the internal die 154 has a sirnilar curve at 10- cations 165 and 167, so that the 1egs of the core 142 are given a reverse bend. In other words, the legs 0f core 142 are bent inward, so that the only fixture required during the heat treating cycle is an internal fixture to rnove the core legs back to the straight configuration, as shown in FIGURE 7. The core 142, after forming and after an internal spreader fixture has been placed in the core window, is stress relieved using C011- ventional heat treating practice which is well-known in the art.

Another method of forming the core is to substitute shearing for the winding, back Winding and cutting method hereinbefore described. The laminations 170 may be sheared while against an angled stop, as shown in FIGURE 22, which, by selecting the proper angle 176 will automatically provide the difference in length of the larninations 170 to provide the desired joint pattern. The end of a continuous coil of oriented magnetic strip material is introduced through the blade opening 174 until the end intersects the angled stop 172. The strip is sheared at point 174. New strip then moves forward over the previously sheared strip until the newly sheared end intersects the stop 172 and the strip is again sheared. This process continues until angled stack of laminations 170 is built up to the desired height E. Tl1is device automatically -cornpensates for variations in strip thickness, since it is the correct angle of shearing, not an absolute length diflerence that is desired. If the angle 1's correct, the proper difference in length of the laminations is auton1atically provided. When the strip gets thicker, it backs up further on'the angled stop 172 and shears With a greater difierence in length. When the strip gets thinner, it proceeds further into the angled stop, shearing with less difference in length.

It is important to get the proper angle 176 so that the outer length or periphery of the core bears the proper relationship to the inner length. If two bends per corner are desired the proper angle is that angle whose tanget is one divided by 6.63. The outer length of a polygonal core having an octagonal shape (2 bends per corner) is 6.63 times the build-up of laminations E 10 internal fixture applied to the core window. Thus, the cores may be heated faster in the stress-anneal cycle and the furnace will use less B.t.u. per hour. Further, stresses are not introduced into the core by end frames and coils. The core configuration is accurate and repeatable, allowing c1ose tolerance coils to be assernbled thereon without pressure on the core. Thus, the core has substantially lower losses than cores of the prior art. Further, the sharp bends at the core corners prevents slipping of the laminations. The larninations retain the position they 0ccupied during the stress anneal cycle. It is easy to assemble the coil on the core, as it is merely necessary to realign the bends to assure that no stresses have been introduced into the core. Further, joint losses are reduced as the joints are locked into position by the sharp discrete corner bends. Adding further to the low losses of the joint, is the fact that there is 110 possibility of having an inco'mplete joint closure. By substantially eliminating stress in the core and preventing incomplete joint closure, cores constructed according 10 the teaching of this invention have been found to have substantial-ly lower losses than core constructions of the prior art. Table I below contains data taken frorn tests comparing cores construc-ted according to the principles of this invention having 5 two discrete bends per corner and no external fixturing during the heat treating process, With cores constructed according to prior art methods wherein the cores are held by caps during the heat treating process. The cores are for use in 25 kva. 2400 Volt transformers and all utilize a stepped-lap joint in one leg of the core having a 20 10 1 ratio to overlap to larnination thickness. The tests were eonducted utilizing an induction of 15.72 kilogausses, both before and after being assembled with coils into complete units.

TABLE I.CORES CONSTRUCTED AOOORDINC%%T(%EEI%%IINCIPLES OF TIE[IS INVENTION VERSUS PRIOR Average T.W. Pereent Average A.W. Percent (True Watts) Unit Increase (Apparent Unit Increase Core Watts) Gote Cores Construeted According to this Invention- 92.6 97. 3 4. 9 130. 2 144. 9 11. 2 Prior Art Cores 94. 0 100. 8 7. 3 133. 7 156. 0 16. 7

Pereent Increase 1. 6 3. 7 2. 6 7. 7

greater than the inner length. For cores with three bends per corner, the proper angle is that angle whose tangent is equal to one divided by 6.43.

Shearing lends itself Well to automation, with the whole core being sheared at once, separated into groups to produce the stepped lap joint, or any other desired joint, and reassem-bled; or, individual groups of laminations may be sheared and assembled to f0rm the core. The Wh0le core may then be die-formed, to form the sharp corner bends, as shown in FIGURE 21 hereinbefore described, or individual groups may be formed and assembled as shown in FIGURE 12.

Another method of forming a core having a plurality of discrete sharp bends at each corner is to Wind and backwind a core as shown in FIGURE 14, die form as shown in FIGURE 21, and stress anneal without cutting the core. The core may be then unwound and sheared into a step-lap pattern, or any other joint design, using the corner bends as a means of locating the incre-mental positions of shearing. The laminations may then be returned to their original stress relieved positions during assembly with the sharp corner bends aligning the laminations and preventing them from shifting out of place.

Wound magnetic cores constructed according to the teachings of this invention have many advantages over wound magnetic cores of the prior art. Expensive external holding fixtures are not required. Accurare cores having a uniform configuration from core to core, straight le-gs and yokes, and square and sharp corners in the core windows, are obtained by merely using a spreader type It can be seen -by examining Table I, that the average core T.W. of the prior art cores was 1.6% greater than the average core T.W. of cores constructed according to the principles of this invention, and the average unit T.W. cf prior art cores (cores completely assembled with end frames and electrical coils into complete units) was 3.7% greater than the average unit T.W. of cores constructed according to the teachings of this invention. The percent increase in T.W. of prior art cores from core -[0 unit was 73%, while the -percent increase from core to unit of cores constructed according to the principles of this invention was 4.9%.

The average core A.W. of prior art cores was 12.6% greater than the average core A.W. of cores constructed aecording to the teachings of this invention, and the average unit A.W. of prior art cores was 7.7% greater than the average unit A.W. of cores constructed according t0 the teachings of this invention. The percent increase of A.W. of the prior art cores from core to unit was 16.7%, while the percent increase cf A.W. of cores constructed according to the teachings of this invention was 11.2%.

Thus, cores constructed accord ing to the principles of this invention not only have a lower T.W. and A.W. than cores of the prior art, but the percent increase in both A.W. and T.W. of cores constructed according to the teachings of this invention, from their condition prior to assembly to the assembled condition, is substantially lower than cores of the prior art.

Since numerous ehanges may be made in the above described apparatus and difference embodiments of the in- 1 1 vention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description and shown in the acco-mpanying drawings shall be interpreted as illustrative, and not in a limiting sense.

I claim as my invention:

1. A magnetic core eomprising a plurality of superposed turns of metallic laminations, said superposed turns of metallic laminations providing a laminated structure in which the outer turn defines a plurality f major outer sides separated by outer corners and the inner turn defines an opening in the structure, at least the outer corners of said structure each having a plurality of discrete bends which provide a plurality of additional outer sides.

2. The magnetic core of claim 1 in which each of the outer corners has two discrete bends.

3. The magnetic core of -claim 1 in which each of the outer corners has three discrete bends.

4. The magnetic core of claim 1 wherein the discrete bends in each of the outer eorners extend to and intersect the inner turn of said structure a predetermined distance apart t0 impart substantially the same configuration t0 all the turns of said structure.

5. The magnetic core of claim 1 wherein the discrete bends at each of the outer corners extend t0 and intersect the inner turn at a common point, providing a structure in which the inner turn has fewer bends than the outer turn.

6. The magnetic core of claim 1 in which the outer turn defines four major outer sides.

7. The magnetic -core of claim 6 in which the opening defined by the inner turn has four major sides, each of which is substantially parallel with one of the major outer sides, forming an inner corner associated With each of the outer corners of the structure, said inner corners having the same nurnber of discrete bends as its associated outer corner.

8. The magnetic core 0f claim 6 in which the opening defined by the inner turn has four sides, each of which is parallel With one of the major outer sides, forming a substantially squared inner corner associated with each of the outer corners 0f the structure.

9. The magnetic core of claim 8 in which the discrete bends of the outer corners repeat through said plurality of superposed lamination turns to form bend lines which meet at its associated substantially squared inner corner.

10. The magnetic core of claim 1 in which each of the turns of metallic larninations are cut to provide single turn laminations having two ends, and at least one openable joint in said structure.

11. The magnetic core of claim 10 in which the turns are disposed to provide an openable joint across a portion of the structure in one 0f its major sides, and an openable joint across the remaining portion of the structure in another of its major sides.

12. The magnetic core of claim 10 in which the two ends of each turn are substantially aligned.

13. The magnetic core of claim 12 in which the aligned ends of at least one of the turns are disposed in one of the plurality of additional sides formed by the discrete bends.

14. The magnetic core of claim 12 in which the aligned ends of each turn are olfset from one another in a predeterrnined stepped relation t0 provide at least one stepped lapped openable joint in said structure.

N0 references cited.

LEWIS H. MYERS, Primary Examiner.

T. J. KOZMA, Assistant Examiner. 

1. A MAGNETIC CORE COMPRISING A PLURALITY OF SUPERPOSED TURNS OF METALLIC LAMINATIONS, SAID SUPERPOSED TURNS OF METALLIC LAMINATIONS PROVIDING A LAMINATED STRUCTURE IN WHICH THE OUTER TURN DEFINES A PLURALITY OF MAJOR OUTER SIDES SEPARATED BY OUTER CORNERS AND THE INNER TURN DEFINES AN OPENING IN THE STRUCTURE, AT LEAST THE OUTER CORNERS OF SAID STRUCTURE EACH HAVING A PLURALITY OF DISCRETE BENDS WHICH PROVIDE A PLURALITY OF ADDITIONAL OUTER SIDES. 